In order to accurately realize vector control and the identification of motor parameters in Variable-Frequency Speed Regulating System, the three-phase stator voltage and DC bus voltage have to be measured in real time. In order to prevent the voltage from being put into the controller"s weak current system, it is necessary to isolate the tested power. Voltage detection circuit with isolation function was designed based on the high precision linear analog optical coupling HCNR201, and it was enclosed in the PCB board of 15 by 32mm. The test results show that the voltage isolation detection circuit is of good linearity and can be applied safely and accurately in ac frequency conversion speed regulating system.